The bovine leukemia virus (BLV) is related only to the HTLV-I and -II retroviruses in genetic structure and DNA sequence. A property which further emphasizes the relationship between these retroviruses is that they contain additional open reading frames (ORFs) located 3' to the envelope gene, which potentially encode transcriptional and immortalizing functions. One of these, the long open reading frame (LOR), has been molecularly reconstructed from a cloned provirus into an expression vector containing differentially regulated promoter units. Co-transfection of the BLV LOR with the BLV long terminal repeat (LTR) establishes that BLV LOR can, in itself, confer high levels of transcriptional activity on the BLV LTR. The human retrovirus HTLV-I has been shown to immortalize T-cells in vitro; however, attempts to immortalize bovine or human blood lymphocytes with BLV have proven negative. We have taken an alternative approach to identify a potential BLV-encoded transforming function by investigating the status of the BLV provirus in cell lines established from BLV-induced B-cell lymphosarcomas. One cell line, NBC-13, has amplified a BLV deletion type provirus approximately 20-fold over single copy genes. The BLV deletion type provirus has been molecularly cloned and sequenced. Results from these analyses show that a protease fusion to the 3' pre-px gene region has occurred and that deletion type proviruses most probably arise from reintegration of spliced BLV cDNA transcripts. The function of this potential BLV gene product with regards to immortalizing properties is currently being assessed.